It's a hard question for me because most of the cooking I do is by technique, tempered and informed by experience.
I do own a lot of books about food, but many of them are reference texts.
I recently bought a copy of Ratio: The Simple Codes Behind the Craft of Everyday Cooking. This is one of the best cooking books (as opposed to the standard cookbook) I've bought in quite a while.
It contains a lot of information in the style of On Food and Cooking with some recipes based on the 33 ratios presented. The idea behind the book is that once you know some key ideas about how to combine certain ingredients, making variations on a theme is easy.
I would highly recommend this book to folks who are looking for something to help them expand their technique and to understand how to vary their favorite recipes while still getting excellent results.